Conventionally, in a constant current circuit (constant current source) using an electropotential difference caused by a source resistor of a current mirror, a startup circuit is provided to surely start a constant current circuit when the supply of a power supply voltage is started so that the startup circuit supplies a startup current to the constant current circuit.
Necessity to Supply a Startup Current to a Constant Current Circuit
FIG. 5 illustrates an example of a constant current circuit using an electropotential difference caused by the source resistor of a current mirror. This constant current circuit 101 includes N-channel MOS transistors N1 and N2, P-channel MOS transistors P1 and P2, and a resistor R1. The N-channel MOS transistor N1, the P-channel MOS transistor P1, and the resistor R1 are connected in series between a supply line L1 and a ground line L2 of a power supply voltage VDD to form a first current path S1 and an N-channel MOS transistor N2 and the P-channel MOS transistor P2 are connected in series to form a second current path S2.
In addition, in the constant current circuit 101, gates of the N-channel MOS transistors N1 and N2 are connected to each other. Gates of the P-channel MOS transistors P1 and P2 are connected to each other. A drain and the gate of the N-channel MOS transistor N2 are connected to each other. A drain and the gate of the P-channel MOS transistor P1 are connected to each other.
By adopting such a circuit configuration, the constant current circuit 101 has a feedback circuit in which the value of a current I flowing through the first current path S1 is identical to the value of a current I flowing through the second current path S2 by a first current mirror circuit including the N-channel MOS transistors N1 and N2 and a second current mirror circuit including the P-channel MOS transistors P1 and P2. The current I flowing through the first current path S1 is referred to below as the “left side current mirror current IL” and the current I flowing through the second current path S2 is referred to below as the “right side current mirror current IR”.
In the constant current circuit 101, the transistor sizes of the N-channel MOS transistors N1 and N2 and the resistance value of the resistor R1 are selected so that, for example, the left side current mirror current IL and the right side current mirror current IR become stable at 0.5 μA. However, as illustrated in FIG. 6, when starting from the point at which the power supply voltage VDD is zero volts, the constant current circuit 101 has the stable point (point at which the current is zero) at which no current flows through both current paths in addition to the stable point at which the left side current mirror current IL and the right side current mirror current IR become identical and stable. FIG. 6 indicates, as a stable point A, the point at which the left side current mirror current IL and the right side current mirror current IR become identical and stable and indicates, as a stable point B, the point at which no current flows through both current paths.
Normally, both currents rarely become stable at the point at which no current flows due to noise or imbalance between the two current paths S1 and S2. However, depending on conditions such as a temperature, a process, and a power source, they may become stable at the point at which no current flows and, once they become stable at the point at which no current flows, the constant current circuit 101 never starts again. Therefore, it is necessary to reach the stable point A by forcibly feeding a current through the N-channel MOS transistor N2 or the P-channel MOS transistor P1 only when the supply of the power supply voltage VDD to the constant current circuit 101 is started. The current necessary for reaching the stable point A is referred to as a “startup current”. In addition, the circuit for supplying the startup current is referred to as a “startup circuit”.
Conventional Startup Circuit